There are a number of methods for simultaneously constraining and illuminating strip transparency media. The most common are the rigid frame types of film gates commonly used in motion picture projectors and similar devices. A more sophisticated version of this rigid constraint approach is the glass platen type of configuration wherein the transparency material is forced to conform to a single flat optically transparent surface (typically glass) or alternatively sandwiched between two such surfaces.
Desirably, the film gate should be such that physical contact between the film surface and a friction imparting element is prevented, in order to avoid scratching the film, which could possibly cause a loss of information contained in the image frame. To this end, improvements in film gate structures have led to the development of fluid gates, to which a curtain, or stream, of air is generated in the area of the gate so as to provide a medium by way of which the film may be frictionlessly supported. An example of such an air pressure gate is described in U.S. Pat. No. 3,457,007 to Conroy, and consists of a differential pressure gate for a motion picture projector and has a plurality of peripheral ducts which pass through the walls of the gate aperture. Air pressure is introduced through these ducts in an effort to apply a substantially uniform pressure throughout an air chamber disposed on one side of the photographic film.
U.S. Pat. No. 4,411,503 to Bailey et al is directed to an air platen bearing. The fluid bearing platen for the film gate structure contains a pair of cavities to which a pressurized fluid, such as air, is applied against opposing surfaces of the photographic film. Between each of the respective cavities, and on one side of the film, there is a thermal dynamically non-throttling material, such as porous stainless steel, through which the pressurized fluid from the cavity passes, where it is directed against opposite sides of the film.
All of the above-mentioned air bearing film gates require an external source of air pressure which can be both cumbersome and costly. Another problem that can be encountered with film gates having a rigid transparent platen have the propensity to develop optical interference patterns between transparency materials and optically flat platens unless indexing matching fluids (typically oils) are used. In addition, it is difficult to maintain true flatness of a transparency over the scanned area which can be detrimental to the scanned image quality in systems with limited depth-of-focus.